#### 12th Standard English Medium Physics Reduced syllabus Public Exam Model Question Paper - 2021

12th Standard

Reg.No. :
•
•
•
•
•
•

Physics

Time : 02:45:00 Hrs
Total Marks : 70

Part I

Choose the most suitable answer from the given four alternatives and write the option code with the corresponding answer.

15 x 1 = 15
1. An electric dipole is placed at an alignment angle of 300 with an electric fi eld of 2 × 105 NC-1. It experiences a torque equal to 8 N m. Th e charge on the dipole if the dipole length is 1 cm is

(a)

4 mC

(b)

8 mC

(c)

5 mC

(d)

7 mC

2. Two metallic spheres of radii 1 cm and 3 cm are given charges of -1 x 10-2 C and 5 x 10-2 C respectively. If these are connected by a conducting wire, the final charge on the bigger sphere is

(a)

3 × 10-2 C

(b)

4 × 10-2 C

(c)

1 × 10-2 C

(d)

2 × 10-2 C

3. _______ and Coulomb's law form fundamental principles of electrostatics

(a)

Newton's law of gravitation

(b)

superposition principle

(c)

ohm's law

(d)

Kepler's law

4. The electric potential V as a function of distance x (metres) is given by V = ( 5x2 + 10x -9) volt. The value of electric field at a point x = 1m is

(a)

20 Vm-1

(b)

6 Vm-1

(c)

11 Vm-1

(d)

-23 Vm-1

5. At Infinity (i.e r = ∞), the electrostatic potential (V) is

(a)

(b)

maximum

(c)

minimum

(d)

Zero

6. The direction of electric field at a point the equatorial line due to an electric dipole is

(a)

along the equatorial line towards the dipole.

(b)

along the equatorial line away from the dipole

(c)

parallel to the axis of the dipole and opposite to the direction of dipole moment

(d)

parallel to the axis of the dipole and in the direction of dipole moment.

7. If C is the capacitance of an air filled capacitor and C is the capacitance of dielectric filled capacitor, then

(a)

C'εrC'

(b)

$C'=\frac { C }{ { \varepsilon }_{ r } }$

(c)

$C'=\frac { { \varepsilon }_{ r } }{ C }$

(d)

$C'={ \varepsilon }_{ 0 }{ \varepsilon }_{ r }$

8. Two wires of A and B with circular cross section made up of the same material with equal lengths. Suppose RA = 3 RB, then what is the ratio of radius of wire A to that of B?

(a)

3

(b)

$\sqrt3$

(c)

$\frac{1}{\sqrt3}$

(d)

$\frac{1}{3}$

9. A wire connected to a power supply of 230 V has power dissipation P1. Suppose the wire is cut into two equal pieces and connected parallel to the same power supply. In this case power dissipation is P2. The ratio $\frac{P_2}{P_1}$ is

(a)

1

(b)

2

(c)

3

(d)

4

10. A piece of copper and another of germanium are cooled from room temperature to 80 K. The resistance of

(a)

each of them increases

(b)

each of them decreases

(c)

copper increases and germanium decreases

(d)

copper decreases and germanium increases

11. In Joule’s heating law, when I and t are constant, if the H is taken along the y axis and I2 along the x axis, the graph is

(a)

straight line

(b)

parabola

(c)

circle

(d)

ellipse

12. Two identical coils, each with N turns and radius R are placed coaxially at a distance R as shown in the fi gure. If I is the current passing through the loops in the same direction, then the magnetic field at a point P which is at exactly at$\frac { R }{ 2 }$ distance between two coils is

(a)

$\frac { 8N{ \mu }_{ ° }I }{ \sqrt { 5 } R }$

(b)

$\frac { 8N{ \mu }_{ ° }I }{ { 5 }^{ \frac { 3 }{ 2 } }R }$

(c)

$\frac { 8N{ \mu }_{ ° }I }{ { 5 }R }$

(d)

$\frac { 4N{ \mu }_{ ° }I }{ \sqrt { 5 } R }$

13. In an electrical circuit, R, L, C and AC voltage source are all connected in series. When L is removed from the circuit, the phase difference between the voltage and current in the circuit is $\frac{\pi}{3}$. Instead, if C is removed from the circuit, the phase difference is again  $\frac{\pi}{3}$.  The power factor of the circuit is

(a)

1/2

(b)

1/$\sqrt2$

(c)

1

(d)

$\sqrt3$/2

14. The ratio between the first three orbits of hydrogen atom is

(a)

1:2:3

(b)

2:4:6

(c)

1:4:9

(d)

1:3:5

15. The charge of cathode rays is

(a)

Positive

(b)

negative

(c)

neutral

(d)

not defined

16. Part II

Answer any 6 questions. Question no. 16 is compulsory.

6 x 2 = 12
17. How does the conductivity of a semiconductor change with rise in temperature? Explain.

18. Two wires A & B are of the same metal of of same length have area of cross section in the
ratio of 2 : If the same potential difference is applied across each wire. What will be the retro of the circuit flowing in A & B?

19. What are the causes for earth's magnetic field according to Gover?

20. State Right hand thumb rule.

21. Define tesla?

22. What is meant by AC vollage.

23. Part III

Answer any 6 questions. Question no.27 is compulsory.

6 x 3 = 18
24. Calculate the electrostatic force and gravitational force between the proton and the electron in a hydrogen atom. They are separated by a distance of 5.3 × 10-11 m. The magnitude of charges on the electron and proton are 1.6 × 10-19 C. Mass of the electron is me = 9.1 × 10-31 kg and mass of proton is mp = 1.6 × 10-27 kg.

25. Consider the charge configuration as shown in the figure. Calculate the electric field at point A. If an electron is placed at points A, what is the acceleration experienced by this electron? (mass of the electron = 9.1 × 10-31 kg and charge of electron = −1.6 × 10-19 C)

26. Derive an expression for electrostatic potential energy of the dipole in a uniform electric field.

27. Determine the number of electrons flowing per second through a conductor, when a current of 32 A flows through it.

28. Obtain the condition for bridge balance in Wheatstone’s bridge.

29. The repulsive force between two magnetic poles in air is 9 x 10-3 N. If the two poles are equal in strength and are separated by a distance of 10 cm, calculate the pole strength of each pole.

30. Part IV

5 x 5 = 25
1. A small ball of conducting material having a charge +q and mass m is thrown upward at an angle θ to horizontal surface with an initial speed vo as shown in the figure. There exists an uniform electric field E downward along with the gravitational field g. Calculate the range, maximum height and time of flight in the motion of this charged ball. Neglect the effect of air and treat the ball as a point mass.

2. Two conducting spheres of radius r1 = 8 cm and r2 = 2 cm are separated by a distance much larger than 8 cm and are connected by a thin conducting wire as shown in the figure. A total charge of Q = +100 nC is placed on one of the spheres. After a fraction of a second, the charge Q is redistributed and both the spheres attain electrostatic equilibrium.

(a) Calculate the charge and surface charge density on each sphere.
(b) Calculate the potential at the surface of each sphere.

1. Show that for a straight conductor, the magnetic field
$\overset { \rightarrow }{ B } =\frac { { \mu }_{ ° }I }{ 4\pi a } (cos\varphi _{ 1 }-cos\varphi _{ 2 })\hat { n }$
$=\frac { { \mu }_{ ° }I }{ 4\pi a } (sin{ \theta }_{ 1 }+sin{ \theta }_{ 2 })\hat { n }$

2. Let E be the electric field of magnitude 6.0 × 106 N C-1 and B be the magnetic field
magnitude 0.83 T. Suppose an electron is accelerated with a potential of 200 V, will
it show zero deflection?. If not, at what potential will it show zero deflection.

1. A conductor of linear mass density 0.2 g m-1 suspended by two flexible wire as shown in figure. Suppose the tension in the supporting wires is zero when it is kept inside the magnetic field of 1 T whose direction is into the page. Compute the current inside the conductor and also the direction of the current. Assume g = 10 m s-2

2. A straight metal wire crosses a magnetic field of flux 4 m Wb in a time 0.4 s. Find the magnitude of the emf induced in the wire

1. A closely wound coil of radius 0.02 m is placed perpendicular to the magnetic field. When the magnetic field is changed from 8000 T to 2000 T in 6 s, an emf of 44 V is induced. Calculate the number of turns in the coil.

2. An inverter is common electrical device which we use in our homes. When there is no power in our house, inverter gives AC power to run a few electronic appliances like fan or light. An inverter has inbuilt step-up transformer which converts 12 V AC to 240 V AC. The primary coil has 100 turns and the inverter delivers 50 mA to the external circuit. Find the number of turns in the secondary and the primary current.

1. Write down the equation for a sinusoidal voltage of 50 Hz and its peak value is 20 V. Draw the corresponding voltage versus time graph.

2. A capacitor of capacitance $\frac { { 10 }^{ -4 } }{ \pi } F$. an inductor of inductance $\frac { 2 }{ \pi }$H and a resistor of resistance 100 Ω are connected to form a series RLC circuit. When an AC supply of 220 V, 50 Hz is applied to the circuit, determine
(i) the impedance of the circuit
(ii) the peak value of current flowing in the circuit
(iii) the power factor of the circuit and
(iv) the power factor of the circuit at resonance.